Techniques of causing electric discharge to occur in gas to produce ions are widely known as ion generating techniques. Microwaves and electron beams are employed to make electric discharge occur.
On the other hand, the laser ion source employing a laser by definition is designed to irradiate a target with a condensed laser beam to evaporate the element of the target and ionize it to generate plasma. The laser ion source is a device for transporting the ions intact as they are contained in the plasma and producing an ion beam by accelerating the ions at the time of extracting the ions (see, for example, Japanese Patent Publication No. 3,713,524 and Japanese Patent Application Laid-Open Publication No. 2009-37764, the entire content of which is incorporated herein by reference). Thus, the laser ion source can generate ions by irradiating a solid target with a laser beam and hence is advantageous when generating a pulsed high current for multivalent ions.
The ions generated by the laser ion source have an initial velocity in a direction perpendicular to the surface of the solid target. Therefore, the laser ion source can be made to transport ions by extending the transport pipe showing an electric potential same as the ion generating section downstream as viewed in the transporting direction. Furthermore, the laser ion source can be so arranged as to block unnecessary ions from passing by setting up electrodes on the plasma transport route and applying a positive electric field to the plasma on the plasma transport route (see Japanese Patent Application Laid-Open Publication No. 2012-99273 the entire content of which is incorporated herein by reference).
Meanwhile, Review of Scientific Instruments 81, 02A510 (published in 2010) (to be referred to as Literature 1 hereinafter), the entire content of which is incorporated herein by reference, describes a technique for the laser beam injection system of the laser ion source. With the technique, the laser beam emitted from a YAG laser is led to a vacuum container from the outside of the vacuum container by way of two mirrors. The laser beam is introduced into the vacuum container through a vacuum window of the vacuum container. In the vacuum container, the laser beam is reflected by a mirror and made to enter a lens. The laser beam is then condensed by the lens and irradiated onto a target.
The laser beam injection system described in Literature 1 requires axial alignment of the mirror and the lens in the vacuum container. Since the mirror and the lens of the optical system are arranged in the vacuum container, the use of a drive mechanism such as a motor is necessary to adjust the relative axial positions of the mirror and the lens from the outside of the vacuum container. Therefore, the laser ion source of the above-cited Literature 1 is accompanied by a problem of a complex structure involving wirings drawn outside the vacuum container.
With the laser ion source as described in the above-cited Literature 1, when a mirror and a lens are arranged in the plasma generation section thereof, they can be stained by laser ablation particles adhering to them to consequently degrade the target irradiation performance of the laser beam. Then, the mirror and the lens may need to be replaced and a stain prevention mechanism (of arranging a transparent film reel and a take-up reel in the vacuum container, for example) may have to be provided.
Therefore, with such a laser ion source, the axial position of the optical system needs to be adjusted again after replacing the mirror and the lens. Additionally, a problem of a complex structure arises when a stain prevention mechanism is arranged in the vacuum container.
Thus, the problem to be solved by the present invention is to provide a laser ion source in which the condenser lens can be axially aligned with ease and which has a simplified structure and also to provide heavy particle beam therapy equipment employing such a laser ion source.